Glascock joined Jethro Tull in 1976, after meeting
Ian Anderson when Carmen opened several dates for Jethro Tull on the
War Child tour. Tull's guitarist,
Martin Barre, said in the "Classic Artists Series" documentary that Glascock's love for the band was unique, in that he was a fan of Jethro Tull who became a member of the band. He replaced Anderson's longtime friend, bassist
Jeffrey Hammond. Glascock played on the Jethro Tull albums ''
Too Old to Rock 'n' Roll: Too Young to Die! (1976), Songs from the Wood (1977), Heavy Horses (1978), Bursting Out (1978), and on three tracks on Stormwatch'' (1979). He received no writing credits in Jethro Tull, but sang backing vocals and became the band's first harmony vocalist. He also played electric guitar on stage when Anderson's arrangement required it, as in performances of "
Skating Away on the Thin Ice of the New Day" on the
Bursting Out live album, in which the rest of the band also played other instruments. Glascock's health problems became apparent when he was unable to complete the 1979 US leg of the
Heavy Horses tour, missing a transatlantic
simulcast from
Madison Square Garden. He played his last concert with Jethro Tull on 1 May 1979, in San Antonio, Texas, three years to the day after his first concert with the band. Despite a diagnosis of heart valve damage caused by an infection, he continued his previous lifestyle, which involved heavy drinking,
marijuana usage and wild parties, and his health continued to deteriorate. Anderson gave him several warnings before laying him off with pay during the production of
Stormwatch (1979) and completing most of the bass guitar parts himself. He learned of Glascock's death in November 1979 during the US
Stormwatch tour, with replacement bass player
Dave Pegg, and had to break the news to the rest of the band. Glascock's close friend,
drummer Barriemore Barlow, was devastated and left the band at the end of the tour. The resulting dislocation, Anderson's own artistic vision, and a decision from the label, prompted a complete overhaul of the lineup, leaving only Anderson and
Martin Barre as full members. Glascock's touring replacement on bass, Dave Pegg, was retained for the band's follow-up album in 1980 and remained with the group until 1995. ==Personal life==